SDA-Decomp

Accurate offset-to-angle decomposition in the presence of steep dips

Accurate reservoir property prediction is highly dependent on the quality of the seismic data amplitude information as a function of incidence angle.

Transformations from offset to angle domain must take account of geologic dip in areas where horizons are not flat: i.e., where a 1D earth model assumption is invalid. Conventional "analytic" approaches can break down in the presence of steeply dipping events. As a rule of thumb, this breakdown occurs when the geologic dip plus the maximum angle required for inversion exceeds 70°. The result can be the introduction of spurious amplitude information to subsequent inversions, leading to poor matches between surface seismic and well data, or misleading reservoir property estimation.

SDA-Decomp is a new approach offered by WesternGeco to provide accurate offset-to-angle transformation where dip is high and large angles are required. The technique involves full ray tracing through a dip-field model of the seismic data to determine the correct offset-to-angle relationship. A summary of the workflow is shown below.

SDA-Decomposition can be applied to depth- or time-imaged gathers. However caution is advised when steep dips and strong lateral velocity variations are present and prestack time-migrated data are to be used. In this case, the basic assumptions of prestack time imaging may no longer be valid, and a full depth imaging sequence should be used instead.